As an ambient temperature is lower, the demand for air-conditioning heating capacity is increasing. A current single-stage compression heat pump can only be started normally at minus 20 degrees, and the air-conditioning heating capacity is severely degraded in a low-temperature environment, the heating effect cannot be ensured, and the reliability of an air-conditioning unit has also been severely tested.
Compared with a single-stage compression heat pump, a two-stage compression system with enhanced vapor injection has a larger heating capacity at a lower temperature and higher energy efficiency; and the two-stage compression system can reduce the pressure ratio of a single-stage compressor and reduce the exhaust temperature. Meanwhile, the suction efficiency and the compression efficiency may be increased, thereby increasing the heating capacity and the heating efficiency.
The two-stage compression system with enhanced vapor injection has two or more cylinders which are divided into a high-pressure stage and a low-pressure stage, wherein a low-pressure cylinder is used for a first-stage compressor, and a high-pressure cylinder is used for a second-stage compressor. The principle of enhanced vapor injection is to inject a gaseous refrigerant into a suction port of the high-pressure cylinder of the compressor from an enhanced vapor injection port in the middle of the compressor. The injected gaseous refrigerant will be mixed with a refrigerant discharged after being compressed by the low-pressure cylinder and then enter the high-pressure cylinder for compression.
At present, a two-way valve is used to control the enhanced vapor injection of a compressor. The two-way valve only has a switching function. When the compressor is running at a high speed, if the two-way valve is suddenly opened, a refrigerant will enter the compressor from an enhanced vapor injection port in the middle of the compressor and is mixed with low-pressure exhaust gas, the suction volume of a high-pressure cylinder is increased suddenly, and the exhaust of a low-pressure cylinder is also affected by injection resistance, thereby resulting in a sudden increase in a compressor load, and resulting in a great reduction in reliability of the compressor. The operating life of components in the compressor will be affected. On the other hand, the two-way valve cannot accurately control the replenishment amount of gas during the intermediate enthalpy increase because it only has the switching function. Therefore, it is difficult to ensure the continuous and efficient operation of the two-stage compression system.